1. Technical Field
This invention relates to an apparatus and method of translating a spacecraft from an injection orbit to a geosynchronous orbit in a time efficient manner while optimizing the energy required for injecting a particular payload.
2. Discussion
In order to place a spacecraft in geosynchronous orbit about a central body, such as the earth, the spacecraft is first launched into an injection orbit by the spacecraft launch vehicle. From this injection orbit, the spacecraft is translated through a series of orbits to the geosynchronous orbit. In order for the spacecraft to translate from its injection orbit to geosynchronous orbit, propulsion thrusters fire to exert a force on the spacecraft and move it through the transfer orbit.
There are a number of strategies for translating a spacecraft from its injection orbit to geosynchronous orbit. In a first strategy, a launch vehicle injects the spacecraft to an elliptical orbit having an apogee greater than the geosynchronous orbit, defined as a supersynchronous orbit. Once the spacecraft has reached supersynchronous orbit, propulsion thrusters are fired when the spacecraft is in a predetermined orientation and in proximity to apogee or perigee. Firing the propulsion thrusters at apogee to create thrust in the direction of orbital velocity raises perigee, and firing the propulsion thrusters at perigee to create thrust in a direction opposite the orbital velocity lowers apogee. These apogee and perigee firings or burns translate the spacecraft from supersynchronous orbit to geosynchronous orbit. In a second strategy, the spacecraft is injected into an elliptical orbit having an apogee less than the geosynchronous, defined as a subsynchronous orbit. Once the spacecraft is in subsynchronous orbit, the propulsion thrusters are once again fired when the spacecraft is in proximity to apogee or perigee and in a predetermined orientation. Firing at apogee to create thrust in the direction of orbital velocity raises perigee, and firing at perigee to create thrust in the direction of orbital velocity raises apogee. The apogee and perigee burns cause the spacecraft orbit to spiral out to the geosynchronous orbit. Such a spiraling-out mission using a specific type of thruster is described in Meserole, J. "Launch Costs to GEO Using Solar Powered Orbit Transfer Vehicles." American Institute of Aeronautics and Astronautics (AIAA) Paper 93-2219, AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference and Exhibit (Jun. 28-30, 1993).
Because the launch vehicle injects the spacecraft into either a subsynchronous or supersynchronous orbit, the spacecraft must include its own propulsion system to effect a translation from injection to geosynchronous orbit and to perform orientation and other stationkeeping maneuvers. This raises several considerations for selecting a particular injection orbit translation strategy. Ideally, an injection orbit is selected so that the weight of the spacecraft without fuel, the dry weight, is maximized. The dry weight generally includes the weight of the instrumentation and the underlying spacecraft structure for the instrumentation. Optimizing dry weight requires a trade-off between the capability of the launch vehicle, how high above the earth the spacecraft can be launched, and the propulsion system of the spacecraft, the on-board thrusters and fuel carried by the spacecraft to translate from injection orbit to geosynchronous orbit and perform stationkeeping maneuvers. Greater injection orbits, i.e., higher apogees, reduce the amount of propellant expended by the spacecraft propulsion system to achieve geosynchronous orbit. On the other hand, the capability or payload capacity of the launch vehicle decreases with an increase in the targeted apogee altitude, so that a more powerful launch vehicle is required to inject a spacecraft having the same mass to an injection orbit having a higher apogee. Thus, in order to optimize the weight of the spacecraft at arrival in geosynchronous orbit, defined as the beginning of life weight (BOL), there is a trade-off between the capability of the launch vehicle and the amount that the propulsion thrusters need to be fired. Of course, the more that the propulsion thrusters are fired, more propellant mass is required, leaving less mass allocated to useful pay load?
Further adding to the above considerations is that there are two types of spacecraft propulsion thrusters, electric and chemical. Chemical propulsion thrusters provide the required thrust for translating the spacecraft from injection orbit to geosynchronous orbit and are capable of exerting a substantial force on the spacecraft. However, chemical propulsion thrusters expend a great deal of mass (propellant) in achieving a predetermined orbit orientation. Electric propulsion thrusters, on the other hand, create significantly less thrust than the chemical propulsion thrusters, but they expend much less mass (propellant) in doing so. That is, electric propulsion thrusters use propellant (mass) much more efficiently than chemical propulsion thrusters. Using electric and chemical propulsion thrusters to effect translation from injection orbit to geosynchronous orbit is described in Forte, P. "Benefits of Electric Propulsion for Orbit Injection of Communication Spacecraft." American Institute of Aeronautics and Astronautics (AIAA) Paper 92-1955, 14th AIAA International Communication Satellite Systems Conference & Exhibit (Mar. 22-26, 1992). A combined electric and chemical propulsion system is also described in Free, B. "High Altitude Orbit Raising with On-Board Electric Power." International Electric Propulsion Conference Paper 93-205, American Institute of Aeronautics and Astronautics (AIAA)/AIDA/DGLA/JSASS 23rd International Electric Propulsion Conference (Sept. 13-16, 1993).
Because chemical propulsion thrusters exert a much higher force than electric propulsion thrusters, they enable translation from injection orbit to geosynchronous orbit in a substantially shorter period of time than electric propulsion thrusters. Furthermore, current transfer orbit strategies for translating a spacecraft from injection orbit to geosynchronous orbit fail to describe a viable burn strategy using electric propulsion thrusters exclusively to translate the spacecraft to geosynchronous orbit. Moreover, substitution of electric propulsion thrusters in chemical propulsion thrusters transfer orbit strategies would require an unacceptable transfer orbit duration (TOD).
Electric propulsion thrusters also introduce yet another consideration, that of stationkeeping and maneuvering. Because electric propulsion thrusters expend substantially less propellant for a given thrust than chemical propulsion thrusters, and that thrust is relatively low compared to chemical propulsion thrusters, they are more desirable for stationkeeping and on-station maneuvers. Because stationkeeping maneuvers require minimal thrust to reposition the spacecraft, electric propulsion thrusters perform stationkeeping using much less mass (propellant) than chemical propulsion thrusters.
The trade-off remains that by using chemical propulsion systems and chemical transfer orbit strategies to achieve geosynchronous orbit, a substantial portion of the spacecraft mass is allocated to propellant for the chemical thrusters. This mass may be traded into a decreased required launch vehicle capability, or, alternatively, traded for increased payload for the same launch vehicle capability or a combination thereof. However, using electric propulsion thrusters to execute chemical propulsion transfer orbit strategies would result in an unacceptably long transfer orbit duration. Thus, it is desirable to provide a transfer orbit apparatus and strategy using electric propulsion thrusters which provides acceptable transfer orbit duration for a given launch vehicle capability and a given payload.